The influence of matrix stoichiometry on interfacial adhesion in composites for wind turbine applications

  • Ross Minty

Student thesis: Doctoral Thesis

Abstract

It is well known that the fibre-matrix interface plays a key role in defining the mechanical properties of fibre composite materials. The ability to efficiently transfer stress between the matrix and the fibres is critical in ensuring the required performance level needed for advanced composite materials. Stress transfer across the fibre-matrix interface is often reduced to a discussion of ‘adhesion’. Past discussions of thermosetting matrices have typically focussed on the chemistry of thematrix system, specifically the task of maximising the level of chemical bonding between the fibre and the matrix to produce the strongest interface. However, many authors have also commented on the potential for residual radial compressive stresses formed at the interface to be a significant contributor to the strength of the interface. There is still a significant weight of opinion that holds that even if these residual stresses at the interface can contribute to the stress transfer capability, then chemistry and chemical reactions must play an active role in defining their magnitude. As such it was the objective of this thesis to develop an understanding of how chemistry and residual stresses formed at the interface could be interrelated to influence the stress transfer capability of the interface. First an understanding was established of how the amine-to-epoxy group ratio (R) of an amine cured epoxy influences the thermomechanical properties of the matrix. Overall it was shown that the R value had a major influence over all of the thermomechanical properties studied. The glass transition temperature (Tg) was shown to change significantly depending on the R value, with a maximum of 87.3 °C observed at a value (R [approx. equal to] 1.25) slightly above the stoichiometric point.The matrix Tg decreased as the R value deviated from this value, approaching room temperature for the extreme ratios. Above Tg, the linear coefficient of thermal expansion (LCTE) was shown to reach a minimum at the stoichiometric ratio due to this ratio inducing the highest crosslink density. Below Tg, the R value appeared to have a less clear influence. The storage modulus (E’) of the matrix was also shown to be affected by the R value, with the stoichiometric ratio possessing the largest magnitude of E’ for temperatures 20°C above Tg. However, for temperatures 20 °C below Tg the storage modulus decreased in magnitude the closer the R value was to R [approx. equal to] 1.25. This was the ratio measured to possess the largest Tg value and thus the highest temperature at which E’ was plotted relative to the other ratios. The effect of changing the R value on the interfacial shear strength (IFSS) was investigated using the microbond test. This was done in combination with changing the surface chemistry of the glass fibre and purity of the hardener. Results showed the magnitude of IFSS to be significantly affected by the R value, independent of the fibre sizing applied. The chemistry of the fibre sizing was shown to influence the maximum IFSS achievable and the R value at which it would occur, however the magnitude differences were not as significant. From these results it was concluded that the R value of the matrix has a greater influence than the chemistry of the fibre sizing in defining the level of adhesion at the fibre-matrix interface. The changes in IFSS were shown to correlate with Tg and the decrease in the contribution of residual thermal stresses at the interface.However, this contribution only represented a portion of the total IFSS value measured. It was concluded that other mechanisms, such as cure shrinkage stresses, must provide the remaining portion of IFSS shown. To expand upon this, the influence of temperature in combination with the other variables discussed was studied using the microbond test within a thermomechanical analyser. At lower temperature
Date of Award1 Feb 2018
LanguageEnglish
Awarding Institution
  • University Of Strathclyde
SponsorsUniversity of Strathclyde & EPSRC (Engineering and Physical Sciences Research Council)
SupervisorJames Thomason (Supervisor) & Liu Yang (Supervisor)

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